This invention relates generally to actuation systems and more particularly to magnetic actuation systems in printers.
Actuation systems using magnetic actuators such as solenoids have long been employed in printers. A typical example of a system, indicated generally at 10, is shown in FIGS. 1A and 1B. The system 10 is used to selectively engage and disengage a pick arm 12 from a gear 14, as shown in FIGS. 1A and 1 B, respectively.
The system 10 is used, for example, to control a print media feed mechanism of a printer (not shown). When the pick arm 12 is engaged, the rotation of the gear 14 is inhibited, as shown in FIG. 1 A, and print media (not shown) is fed into a print media path, as is known in the art. However, when pick arm 12 is disengaged from gear 14, as shown in FIG. 1B, the gear rotates freely in a clockwise direction. This rotation stops the movement of the media through the print media path.
The pick arm 12 is selectively engaged and disengaged under the control of a solenoid 16. The solenoid 16 exerts an attractive magnetic force upon a magnetic arm 18, to which the pick arm 12 is attached at a distal end. The magnetic arm 18 is preferably a ferromagnetic material such as soft iron or steel but can also be a permanent magnet. The magnetic force opposes the force generated by a spring 20, which biases pick arm 12 into engagement with the gear 14, and the frictional force exerted by the gear 14 against a pick 22 when the pick 22 is engaged with the gear 14. If the magnetic force is strong enough, the pick arm 12 is disengaged from the gear 14 and the gear is permitted to rotate as shown in FIG. 1B. Once the pick arm 12 is disengaged, however, only a small amount of force is required to oppose the force generated by spring 20 and hold the arm 18 against solenoid 16.
Referring to FIG. 2, a plot 24 of force required to move the arm is plotted as a function of distance "X" as measured between the solenoid 16 and the arm 18, referred to as the "gap distance". Two positions are noted on the graph: an engagement position "X.sub.ENG " wherein the pick 22 is engaged with the gear 14; and a solenoid position "X.sub.SOL " wherein the pick 22 is disengaged from the gear 14 and adjacent to the solenoid 16. Superimposed on the graph is a plot 26 of the force exerted on the arm 18 by the solenoid as a function of distance X.
It is apparent from FIG. 2 that there is a mismatch between the force requirements to move the arm and the available force generated by the solenoid. The force required to move the arm is greatest when the pick is engaged at X.sub.ENG. However, the force generated by the solenoid 16 is asymptotically approaching a minimum at this point. Thus, the force produced by the solenoid is inadequate to disengage the pick 22 from the gear 14. A similar but opposite disparity exists at X.sub.SOL where the pick 22 is disengaged from the gear 14 and the arm 18 is adjacent to the solenoid 16. The force required to hold the arm 18 in the solenoid position is minimal. However, the force generated by the solenoid is approaching its maximum.
In order to overcome this disparity, the solenoid must be sized large enough to generate a force F.sub.ENG sufficient to overcome the force required to disengage the pick 22 from the engaged position X.sub.ENG. A graph 26' of the minimum requisite force is shown in FIG. 2. A solenoid necessary to generate this force, however, is more expensive, consumes more power and space, and generates more noise than a smaller solenoid. Moreover, the solenoid is excessive for holding the arm 18 in the solenoid position.
Accordingly, a need remains for a magnetic actuator system that more closely approximates the force requirements of the application. In addition, the actuator system should not consume any additional power than existing magnetic actuator systems.